The present invention relates to a flexible printed wiring board and multilayered flexible printed wiring board, and to a mobile telephone terminal employing this multilayered flexible printed wiring board.
A rapid popularization of mobile telephone terminals (hereinafter referred to also as mobile telephones) has occurred in recent years. With the main objective of compacting the mobile telephone, this has been accompanied by an increased demand for fold-up mobile telephones. FIG. 1 is a schematic diagram of a fold-up mobile telephone. As shown in FIG. 1, the configuration of a fold-up mobile telephone comprises a first case 10, second case 30, and hinge part 20 that turnably connects the first case 10 and second case 30. The fold-up mobile telephone is foldable in the direction of the arrow shown in FIG. 1.
Flexible printed wiring boards (hereinafter also referred to as FPC substrates) are widely employed in mobile electronic devices, and in particular in mobile telephones, because of their excellent flexibility and flexure characteristics.
The FPC substrate referred to here (hereinafter also referred to as a 3-layered substrate) denotes a substrate comprising an adhesive layer on one or both surfaces of an electric insulating layer such as a polyimide film to which a conductor layer such as copper foil is laminate-adhered, or a substrate in which a coverlay comprising an electric insulating layer such as a polyimide film and adhesive layer is additionally laminate-adhered to the laminate-adhered conductor layer described above on which a circuit has been formed. The FPC 2-layered substrate (hereinafter also referred to as a 2-layered substrate) in the description denotes a substrate in which an electric insulating layer of polyimide or the like is coated and then cured on a conductor layer such as copper foil or the like, or a substrate in which a coverlay is additionally laminate-adhered on the conductor layer on which a circuit has been formed. The multilayered flexible printed wiring board (hereinafter also referred to as a multilayered FPC substrate) in the description denotes a substrate in which a plurality of FPC substrates or 2-layered substrates are laminate-adhered by way of an adhesive layer in 2 or more layers in a range of variations.
FIG. 2 shows one usage example of an FPC substrate in a mobile telephone. As shown in FIG. 2, the role of an FPC substrate 200 is to electrically connect a circuit substrate 100 contained in the case 10 and a circuit substrate 300 contained in the case 30.
An increase in the wiring density of the circuit substrates themselves assembled in mobile telephones and a multilayering the circuit substrates has accompanied advancements in function and miniaturization of mobile telephones in recent years. Accordingly, there has been an increase in the instance of the multilayering of, for example, the circuit substrates 100, 300 contained in the cases 10, 30, and of the connecting of the case 10 and case 30 by the FPC substrate 200 based on the connection of a plurality of FPC substrates.
Methods for the connection thereof include a method based on the separate and independent installation of the circuit substrate 100 and circuit substrate 300 and the use of a connector for connecting the two substrates by way of the FPC substrate 200, and the use of a multilayered FPC substrate in which the circuit substrates 100, 300 and FPC substrate 200 are integrated.
FIG. 3 shows one example of a multilayered FPC substrate.
As shown in FIG. 3, a multilayered FPC 400 is configured from the circuit substrate 100, circuit substrate 300, and the FPC substrate 200 positioned between the circuit substrate 100 and circuit substrate 300. Moreover, while not shown in the diagram, the circuit substrate 100 of FIG. 3 has a layered configuration identical to the circuit substrate 300 but with a different circuit pattern. For example, while if the circuit substrate 300 of FIG. 3 has a 6-layered configuration the circuit substrate 100 also has a 6-layered configuration, the circuit substrates 300 and 100 have different circuit patterns.
The circuit substrates 100 and 300 are configured by the laminate-adhering of a plurality of coverlay-affixed FPC substrates 70 (hereinafter simply referred to as the FPC substrate 70) and adhesive sheets 60. The FPC substrate 70 comprises an FPC substrate 40 comprising an electric insulating layer 42, adhesive layer 41 and conductor layer 43 on which a circuit is formed, and a coverlay 50 for covering the conductor layer 43. In addition, the coverlay 50 comprises an electric insulating layer 52 and adhesive layer 51. The FPC substrate 200 generally comprises an FPC substrate 70.
A multilayered FPC substrate constitutes an integrated multilayered FPC substrate that, to achieve a high densification and flattening thereof, does not comprise a connecting part.
In addition, in order to ensure adequate flexibility (flexure characteristics) in the flexure section of an integrated multilayered FPC substrate, the FPC substrates are not adhered in this section and only the circuit substrate section is laminate-adhered in this section.
A configuration in which a non-bonding part is provided in the flexure part of a multilayered flexible circuit substrate to prevent bonding of insulating layers has been disclosed (Japanese Unexamined Patent Application No. H7-312469).
However, a problem inherent to methods for the manufacture of integrated multilayered FPC substrates lies in the fact that, because the laminate-adhering is based on a heat-pressurizing treatment of, for example, 140 to 200° C.×20 to 50 kgf/cm2, the connecting sections of the FPC adhere even without an adhesion sheet or the like and adequate flexibility cannot be maintained (or to put this another way, adequate flex resistance cannot be exhibited). The problem has been conventionally resolved by manually separating the individually adhered FPC substrates. This results in an additional problem of increased manufacturing costs.